Obstructive sleep apnea syndrome (OSA) affects about 5% of adults and has severe adverse consequences for many regulatory systems (cardiovascular, metabolic, cognitive). Active contraction of upper airway muscles allows OSA patients to keep the airway open and breathe adequately during wakefulness but is insufficient during sleep; hence sleep-disordered breathing occurs. Great progress has been made towards the understanding of the mechanisms responsible for the maintenance of upper airway muscle activity during wakefulness and its decline during normal sleep in healthy subjects. However, with the exception of studies with recurrent hypoxia, our understanding of upper airway control in sleep-disordered breathing is limited. In this project, we plan to study how upper airway muscle activity is altered as a result of sleep disruption, sleep loss and circadian misalignment. We find that serotonin type 2A (5-HT ) receptors which mediate wake-related drive to upper airway (hypoglossal-XII) 2A motoneurons have higher levels in the XII motor nucleus at the onset of active period when compared to the rest period, that the magnitude of upper airway muscle activation during both wakefulness and rapid eye movement sleep differs between the rest and active periods, and that it is depressed by sleep loss or when the circadian rhythm of sleep is abolished. This leads us to hypothesize that circadian rhythm, intensity of prior muscle activation, and sleep drive importantly determine the level of activity in upper airway muscles and that 5-HT receptors 2A contribute to this process. Our research plan has three Specific Aims: (1) Measure lingual electromyographic (EMG) activity levels over the entire circadian cycle in normally sleeping rats, rats with circadian regulation of sleep abolished by housing in constant light, and rats subjected to sleep deprivation. We will also determine the time course of lingual EMG changes during state transitions (sleep entries and awakenings) because they represent the critical events during which upper airway obstructions begin and are resolved, and measure the endogenous, 5-HT receptor-mediated drive in XII motoneurons at different phases of the circadian cycle. (2) Determine the 2A normal circadian time course of 5-HT receptor expression in the XII nucleus using quantitative reverse 2A transcription-polymerase chain reaction, Western blots and immunohistochemistry to identify its phase relationship within the circadian cycle and then follow up with mRNA microarrays to identify targets and pathways unique to the XII nucleus. (3) Differentiate between the intrinsic (activity-dependent) and sleep/circadian rhythm-dependent effects on lingual EMG across sleep-wake states in chronically instrumented rats with stimulated use of the tongue and in anesthetized rats with direct activation of XII motoneurons with 5-HT. These studies will, for the first time, quantify upper airway muscle activity across sleep-wake states during normal, disrupted and misaligned sleep, and establish the contributions of circadian regulation, muscle activation, sleep loss, and 5-HT receptors to this control. As such, they will fill a major gap in our understanding of state-dependent control of the upper airway under conditions highly relevant to OSA.